Secondary batteries are devices which store electrical energy in a chemical form and generate electricity when needed. The secondary batteries are also referred to as rechargeable batteries because they can be charged repeatedly. Common secondary batteries include lead accumulators, NiCd batteries, NiMH accumulators, Li-ion batteries, Li-ion polymer batteries, and the like. When compared with disposable primary batteries, not only are the secondary batteries more economically efficient, they are also more environmentally friendly.
Secondary batteries are currently used in applications requiring low electric power, for example, equipments to start vehicles, mobile devices, tools, uninterruptible power supplies, and the like. Recently, as the development of wireless communication technologies has been leading to the popularization of mobile devices and even to the mobilization of many kinds of conventional devices, the demand for secondary batteries has been dramatically increasing. Secondary batteries are also used in environmentally friendly next-generation vehicles such as hybrid vehicles and electric vehicles to reduce the costs and weight and to increase the service life of the vehicles.
Generally, secondary batteries have a cylindrical, prismatic, or pouch shape. This is associated with a fabrication process of the secondary batteries in which an electrode assembly composed of an anode, a cathode, and a separator is mounted in a cylindrical or prismatic metal casing or a pouch-shaped casing of an aluminum laminate sheet, and the casing is filled with electrolyte. Because a predetermined mounting space for the electrode assembly is necessary in this process, the cylindrical, prismatic or pouch shape of the secondary batteries has a limitation in developing various shapes of mobile devices. Accordingly, there is a need for secondary batteries having a new structure that is variously adaptable in shape. To fulfill this need, suggestions have been made to develop flexible linear secondary batteries having a very high ratio of a length and a cross-sectional diameter, hereinafter referred to as cable-type secondary batteries.
However, the flexible cable-type secondary batteries are frequently subject to the external physical impact due to their structural characteristics, for example, the twist thereof, which may result in a short circuit. Further, when Si or Sn is used as an anode active material, the active material may peel off due to the expansion and shrinkage of electrodes caused by the repeated charging and discharging. Also, when an anode active material layer made of a metal, which is used to increase the capacity of batteries, is thick, it is difficult for lithium ions to diffuse in the anode active material layer, thereby substantially restricting the implementation of battery capacity.